Patent Document 1 indicated below discloses a fuel cell system which measures an AC impedance by superimposing a sinusoidal wave onto the output voltage of a fuel cell using a DC-DC converter serving as a voltage conversion unit, and detecting any variation of the sinusoidally-varying power. In this fuel cell system, if the DC-DC converter shows an abnormality, the superimposition of a sinusoidal wave onto the fuel cell output voltage is set so as to be performed by changing the output current of an inverter.